Printing Out User Information

The id utility lists a user's real and effective user-id numbers, real and effective group-id numbers, and the user's group set, if any. id will only print the effective user-id and group-id if they are different from the real ones. If possible, id will also supply the corresponding user and group names. The output might look like this:

$ id
-| uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty)

This information is exactly what is provided by gawk's `/dev/user' special file (see section Special File Names in gawk). However, the id utility provides a more palatable output than just a string of numbers.

Here is a simple version of id written in awk. It uses the user database library functions (see section Reading the User Database), and the group database library functions (see section Reading the Group Database).

The program is fairly straightforward. All the work is done in the BEGIN rule. The user and group id numbers are obtained from `/dev/user'. If there is no support for `/dev/user', the program gives up.

The code is repetitive. The entry in the user database for the real user-id number is split into parts at the `:'. The name is the first field. Similar code is used for the effective user-id number, and the group numbers.

# id.awk --- implement id in awk
# Arnold Robbins, [email protected], Public Domain
# May 1993

# output is:
# uid=12(foo) euid=34(bar) gid=3(baz) \
#             egid=5(blat) groups=9(nine),2(two),1(one)

BEGIN    \
{
    if ((getline < "/dev/user") < 0) {
        err = "id: no /dev/user support - cannot run"
        print err > "/dev/stderr"
        exit 1
    }
    close("/dev/user")

    uid = $1
    euid = $2
    gid = $3
    egid = $4

    printf("uid=%d", uid)
    pw = getpwuid(uid)
    if (pw != "") {
        split(pw, a, ":")
        printf("(%s)", a[1])
    }

    if (euid != uid) {
        printf(" euid=%d", euid)
        pw = getpwuid(euid)
        if (pw != "") {
            split(pw, a, ":")
            printf("(%s)", a[1])
        }
    }

    printf(" gid=%d", gid)
    pw = getgrgid(gid)
    if (pw != "") {
        split(pw, a, ":")
        printf("(%s)", a[1])
    }

    if (egid != gid) {
        printf(" egid=%d", egid)
        pw = getgrgid(egid)
        if (pw != "") {
            split(pw, a, ":")
            printf("(%s)", a[1])
        }
    }

    if (NF > 4) {
        printf(" groups=");
        for (i = 5; i <= NF; i++) {
            printf("%d", $i)
            pw = getgrgid($i)
            if (pw != "") {
                split(pw, a, ":")
                printf("(%s)", a[1])
            }
            if (i < NF)
                printf(",")
        }
    }
    print ""
}

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